The ability to manipulate small droplets drives technology in many important fields that are vital to the well-being and advancement of society including medicine, chemistry, and biology. The droplets include liquid and may be exclusively liquid or include a gel, a colloid, an emulsion or a liquid-coated solid particle. Examples of uses in such fields include screening for drug efficacy and interaction, ecological and biological contaminants, and gene expression. Increasingly, keys to success have been found in reducing the volume of liquid required per operation, increasing the speed of operations, parallelizing operations, and decreasing operation cost. Driven by these requirements, miniature liquid manipulation lab-on-a-chip (LoC) devices, such as the prior art device shown in FIG. 1, have been developed to pump, meter, mix, and separate liquids with volumetric resolution on the order of nanoliters. LoC platforms offer a generic and consistent way to miniaturize, automate and parallelize chemical and bio-chemical processes.
Prior art LoC devices typically handle liquids continuously or in droplets (digital). Digital-fluidic devices manipulate droplets by surrounding them in an immiscible fluid and forcing them through predefined channels; by using surface interactions such as electrowetting, surface acoustic waves (SAW), heterogeneous (textured) sufaces or other means that require the liquid to contact the surface; or by using acoustic radiation generated by arrays of ultrasonic actuators.
Thus, existing LoCs suffer from fundamental limitations of requiring surfaces that must be patterned (e.g., physical channels and/or chemical traces, such as hydrophobic/hydrophillic coatings, to direct transport of droplets) or equipped with actuators (e.g., electrodes and piezo-electric materials for SAWs and acoustic levitators), and/or the use of a second carrier liquid. These requirements add cost and complexity. Most LoCs also require the manipulated droplet to contact the surface of the device or the carrier fluid, which raises the possibility of contamination and the need for chemical and physical (e.g., wetting angle for solid surfaces and insolubility for carrier fluids) compatibility between materials.
To put the present disclosure in context, we first provide a brief review of related work on microfluidic manipulation, with a focus on techniques used in LoC devices and then discuss work on gas-layer-mediated interaction of droplets with surfaces.
Microfluidics
Microfluidics is the study of fluids at small length and volume scales, typically in amounts less than 1 microliter, which corresponds to a droplet diameter of slightly more than a millimeter. The amount of work in the field has grown dramatically in the last 15 years. Much of the effort in microfluidics is concerned with the manipulation of liquids, the most common operations being transport, mixing, and segregation. Fluid is processed as continuous streams contained in small pipes or channels or as droplets which are manipulated on planar surfaces and surrounded by a second immiscible fluid which can be a liquid or a gas. Hybrid methods also exist in which droplets are formed and processed within channels. The most important technological applications of microfluidic manipulation are in LoC devices. LoC devices are miniature laboratories, typically planar with overall dimensions on the centimeter scale, that combine the abilities to aliquot, separate, mix, and transport fluids with dedicated sensors that measure a wide variety of physical, chemical, and biological properties using optical and electronic means. We next describe existing fluid handling methods used in LoC devices and their limitations, so that the potential advantages and novelty of the microfluidic manipulation method disclosed herein can be placed in context.
Flows
Many prior art LoC devices handle fluids in a conventional macroscopic-way using closed channels or pipes. Rather than connecting together many tiny tubes, small channels are instead formed in the surface of a material using lithography-based approaches, shown for example in FIG. 1. Presently, most devices are fabricated from the silicon compound polydimethylsiloxane (PDMS) using a technique known as soft-lithography. After etching, closed channels are formed by bonding a second material, usually glass, to the etched side of the original surface. Channel widths as small as 50 nm are possible using soft-lithographic techniques. All flow-based technologies must deal with the problem of surface contamination, which can occur during fabrication and assembly or from residual material from a previous use of the surface. Even on the most liquid-phobic surfaces (e.g., hydro- or oleophobic), a film of liquid remains after the bulk of the fluid has passed or been removed, enabling potential contamination. Additionally, continuous flow devices must be partially or completely filled before they are functional which necessarily requires more material.
For fluids to flow in microchannels, forces must be applied to counteract viscous drag. Typically, pressure differences are created across the channels to induce Poiseuille flow, Pressures are frequently created using sources of compressed gas, or pumps, which can be external or integrated in the LoC device. Pressure differences can also be achieved by rapidly rotating a circular LoC with a primarily radial channel geometry centered on the axis of rotation. Pressure driven flows have the disadvantage that volumetric flow rates depend on the impedance of the channel, which can change if residues build up on the channel walls.
Flows are also driven by linear actuators (e.g., motorized syringes) which dispense fluid at fixed volumetric flow rates. However, unless each channel is driven by a dedicated actuator, flow rates in individual channels will differ due to impedance differences associated with varying channel length and cross-sectional area. An additional disadvantage is the need to connect the external actuators to the LoC, as shown in FIG. 1.
The last common method of driving flows in channels is electrokinetics, which makes use of electric fields and charges, either intrinsic (e.g., from ionic solutions) or dipoles which can either be intrinsic or induced. There are many variations on this technique, with electroosmotic flow (EOF) being arguably the most prevalent. In EOF driving, an electric potential is applied between the insulated walls of the channel and the fluid which induces charge separation at the wall. An electric field applied along the length of the channel causes the charges in the fluid to move. The charges moving in the fluid drag the surrounding fluid and the fluid interior to the channel due to viscous coupling. Aside from the surface contact problems associated with all channel flows, electrokinetic flows also have to contend with resistive heating, pH gradients caused by the applied fields, and the potential for bubble generation and solids buildup at the driving electrodes due to electrolysis.
Droplets
A competing technique to continuous flows is the handling of materials in droplet form. This method, known as digital-microfluidics, surrounds droplets with an immiscible fluid (gas or liquid) and shuttles them about on or above a surface using various physical forces realized through heterogeneous surface structures. There is some overlap between continuous flow and droplet manipulation methods, in particular, a technique known as segmented flow microfluidics in which plugs of fluids or droplets are dispersed in a surrounding immiscible fluid. However, as the limitations associated with closed channels and their various driving mechanisms are the same, we do not discuss this method further. Below we describe some common droplet manipulation techniques used in LoC devices, all of which restrict manipulation to patterned areas and, except as noted, require surface contact.
Electrowetting
When a liquid contacts a surface, a contact line is formed at the liquid/fluid/solid interface that is characterized by the angle φc between this interface and the plane of the surface. The “contact angle” is a function of the intermolecular forces and is low when the liquid molecules are more strongly attracted to the surface than each other and high (e.g., water on wax) when the liquid molecules are more strongly attracted to each other than the surface molecules.
The wetting angle can be modified by many factors, including the electrical potential difference between the fluid and the surface. Increasing the potential difference between the liquid and the substrate reduces the contact angle, which causes the edges of the droplet to move outward. The effect can be exploited to move droplets on a surface patterned with multiple electrodes and covered by a common single transparent electrode on the top. FIG. 2, which is reprinted from Article A (see the listing of Articles hereinbelow), shows such a prior art device with square electrodes. The electrode beneath the droplet on the far left of FIG. 2 is energized while the surrounding eight electrodes are at the same potential as the droplet. When an adjacent electrode is energized and the one beneath it de-energized, the droplet flows onto the energized electrode. The image also shows droplets being split and aliquoted from a reservoir.
By repeating the basic voltage pattern, electrowetting can move droplets wherever electrodes have been placed. However, since droplets contact the surface, this technique is at risk for contamination and interaction with residual films of fluid. Similarly, the different wetting properties of different liquids (for example, different blood samples) influence the transport properties. For example, it takes longer for a droplet with higher viscosity to transfer between electrodes, and if this time is significantly longer than the switching time, it could cause the drop to be split or lose spatial coherence with the driving signal.
Surface Acoustic Waves (SAW)
Surface acoustic waves (SAW) are elastic waves that propagate along solid surfaces. In LoC devices, SAWs have amplitudes on the order of nanometers and are typically generated by applying interdigitated electrodes to a piezoelectric surface. When a potential difference is applied between the electrodes, the surface either expands or contracts depending on its structures and the sign of the voltage. Driven at high voltage and with frequencies tuned to match the surface dispersion relation, traveling surface waves are emitted.
Two types of SAW-based manipulation have been explored: traveling surface acoustic wave SAW (TSAW) and standing surface acoustic wave (SSAW). Both make use of liquid-phobic coatings to reduce wetting and the forces required to move droplets. In TSAW, a single piezoelectric actuator generates a propagating wave that travels away from the device. When the wave encounters a droplet resting on the surface it is strongly attenuated and transfers its mechanical energy into the nearest edge of the droplet, generating an internal circulation via acoustic streaming which drives the drop in the direction of wave propagation, as shown in the prior art FIG. 3, which is reprinted from Article B. In SSAW, two actuators are used to generate a standing wave between them. Droplets in this field move to nodes where energy transfer is minimal. Droplets are moved by varying the relative phase of the two waves which shifts the positions of the nodes and thus the droplets.
LoC implementation of SAW methods can move droplets at relatively high speeds and are inexpensive to manufacture, due to the use of SAWs in numerous consumer electronic applications and this technique is subject to contamination. However, transducer density is limited due to unwanted wave interference, and the droplet paths are limited by the locations and orientations of the actuators.
Anisotropic Surfaces
As described above, electrowetting manipulates droplets by using an electric field to create gradients in surface energy at their edges, causing them to move. Other techniques for achieving gradients in contact line surface energy rely on permanent chemical or mechanical alteration of the surface. In one method, a gradient in the interfacial energy was produced by depositing a hydrophobic silane film whose density increased in one direction along the otherwise hydrophilic substrate. A droplet placed on the hydrophobic end spontaneously moved to the hydrophilic end but at an undesirably slow speed due to an effect known as contact angle hysteresis. To increase the speed, the substrate was vibrated horizontally which resulted in a “rectified” motion in which the droplet's trailing edge moved forward but, due to the hysteresis, did not cause its leading edge to move backwards. In a related approach, surface anisotropy was realized by locally varying the surface density of small hydrophobic surface posts on which the droplet sits, as shown in the prior art FIG. 4, which shows a patterned surface to drive a droplet (superimposed image) using surface energy inhomogeneity, and is reprinted from Article C. Droplets are pinned within a local energy gradient minima, but can be moved in a single direction (rectified motion) by symmetric horizontal substrate vibration.
Levitation
Non-surface-contacting droplet transport can be achieved via the Leidenfrost effect, in which droplets are levitated on a layer of gas due to droplet evaporation. The Leidenfrost effect can be observed, for example, in the skittering of water droplets introduced onto a hot pan whose temperature exceeds approximately 200° C. When a droplet approaches a sufficiently hot surface it immediately begins to evaporate. The evaporating gas of the droplet along the heated surface causes the droplet to levitate above the surface and skitter around until it completely evaporates. Although not yet employed in LoC devices, authors have recently described methods of controlling Leidenfrost droplet motion using asymmetric sawtooth-like patterning of the surface. When placed on a patterned surface, droplets travel in the direction where they are descending the less steep portion of the sawtooth-like surface and can even be made to climb up the surface when it is inclined. By adding additional features to the surface grooves, droplets can be made to turn relative to the grooves as the substrate temperature is varied. Although Leidenfrost droplet manipulation does not require surface contact, it does require surface patterning, which restricts possible movement directions, and, most seriously, material is lost and droplet lifetime limited by the necessity of evaporation to produce the gas layer that drives levitation. Leidenfrost droplet manipulation also disadvantagously requires heat, and the heat may detrimentally affect the liquid droplet in other ways.
Another non-contact method that is rapidly developing uses acoustic radiation forces to manipulate droplets in LoCs (see Article C). This method is similar to the SAW technique, but generates high intensity standing waves in air using an array of ultrasonic actuators embedded in the surface and a reflector placed above the surface. Material is suspended in acoustic nodes whose positions vary with the size, density, and compressibility of the suspended material. In static devices, material collects at acoustic nodes, while in dynamic devices the acoustic radiation field is changed in time to pass objects between elements, similar to the electrowetting devices described above. Acoustic methods are inherently non-surface-contacting and chemically compatible with most materials. However, their operation is sensitive to physical properties of the manipulated material and requires synchronized arrays of transmitters which limit spatial resolution and increase device complexity. A related technique that uses four ultrasonic actuators to excite short wavelength modes in a surface formed from a flexible plate does not require large arrays of transmitters (see Article CC), but is limited in its ability to manipulate objects by the modal structure of the plate.